Study of near scrape-off layer (SOL) temperature and density gradient lengths with Thomson scattering

Abstract

Improvements to the Thomson Scattering diagnostic have enabled the study of near scrape-off layer (SOL) decay lengths in the 2014 ASDEX Upgrade experimental campaign. A database of H-mode discharges has been studied using a two-line fit method for the core and log-linear fit for the near SOL region under both attached and detached divertor conditions. SOL electron temperature profiles have been found to have a radial exponential decay distribution and which does not vary poloidally, consistent with the two-point model. In attached H-mode regimes, a log-linear regression shows that the SOL upstream dataset has the same main parametric dependencies as the scaling inferred from downstream Infrared camera measurements. A simple collisional relation from two-point model is found to best relate the upstream decay lengths and downstream divertor power widths. The SOL gradient length appears to be independent of pedestal parameters, but may correlate with the pedestal electron pressure parameters. Both the pedestal and SOL density and temperature scale lengths are linearly correlated with an almost constant gradient ratio,. The smaller gradient ratio and the fact that the Spitzer-Härm model is more valid, agrees with the studied plasma lying in the highly collisional regime. The high density transition in L-mode plasmas as reported in many machines has been observed in AUG detached H-mode regimes. When the flattening of density profile happens in H-mode detached plasma, the broadening of near SOL decay length , also appears which may be good news for the future machine.

Study of near SOL decay lengths in ASDEX Upgrade under attached and detached divertor conditions

“…Classical heat conduction relates ∥ and the temperature decay width, , through the well know results ∥ , . Previous study of H-mode in attached condition showed that this relation is consistent with the upstream Te and target IR data indicating a small broadening (not detectable within experimental errors) of this part of the SOL by radial transport [12]. The electron temperature, , profiles in the near SOL have been found to have a radial exponential decay distribution which, moreover does not vary poloidally.…”

Turbulent heat transport in TOKAM3X edge plasma simulations

“…A recent multi‐machine empirical scaling law for the H‐mode found λ q , targ ∝ and has predicted λ q , ITER ≈ 1 mm, raising concern for future reactor devices. Moreover, the B pol functional dependence has been retrieved in other H‐mode experiments and is observed even for the L‐mode both in limited and divertor configurations, suggesting some universality in the underlying mechanisms.…”

Analytical calculations for impurity seeded partially detached divertor conditions

“…From the divertor entrance situated at the target distance L div to the mid-plane, the narrow power width λ q is supposed to be valid, which is about 2 mm in AUG at I p = 1 MA and predicted to be slightly below 1 mm in ITER [7]. The power width λ q derived from IR measurements at the target and the validity of Spitzer conductivity could recently be confirmed by Thomson scattering measurements of the T e width in the outer midplane using the relation λ q = 2 7 λ T e [25]. At the target, a broadened power width λ int is measured due to the effect of the power spreading S, via λ int ≈ λ q + 1.64S [22].…”